1. Field of the Invention
The invention relates generally to the field of semiconductor fabrication. More particularly, the invention relates to forming gate oxides of different thicknesses.
2. Discussion of the Related Art
Prior art transistor gate oxides are well known to those skilled in the art. In the past, gate oxides of different thicknesses have been formed on a single wafer.
A conventional way to achieve two different gate oxide thicknesses on a single wafer is to grow an oxide with certain thickness and selectively etch certain regions. A second oxide growth is performed after the etch. Regions that were previously etched will exhibit a thinner resulting oxide. Those regions that were not etched will exhibit a thicker resulting oxide.
Another conventional way to vary the gate oxide thickness on a single wafer is to implant nitrogen into the gate region before gate oxidation. The oxidation growth rate depends on the nitrogen implant dose. By selectively exposing some gate regions to the nitrogen implantation, different oxide thickness can be obtained.
The first art approach has a surface cleanliness problem due to direct contact of the gate oxide with photoresist. The second approach also has a uniformity problem with regard to nitrogen dopant concentration. Variations in nitrogen concentration cause unwanted variations in the resulting gate oxide thicknesses. The second approach also has a problem with regard to stray nitrogen in the channel region, (which is near the gate oxide region) degrades carrier mobility.
Heretofore, with regard to formation of gate oxides of different thicknesses, the requirements of cleanliness, uniformity, and avoidance of carrier mobility degradation have not been fully met. What is needed is a solution that simultaneously addresses all of these requirements.